Web treatment apparatus and methods

ABSTRACT

A web with a plastic surface is dislcosed as being exposed to an a.c.-produced corona thereby to increase its printability and coatability. As a result of such treatment the web surface undesirably acquires patterns of charge. A circuit technique is disclosed for so erasing such charge patterns that the web surface is assuredly left substantially uncharged.

United States Patent [1 1 Gibbons et al.

[451 Sept. 4, 1973 WEB TREATMENT APPARATUS AND METHODS [75] Inventors:Carl B. Gibbons; William C. Kerr;

Roger H. Maddocks, all of Rochester, NY.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

22 Filed: Mar. 30, 1972 21 Appl. No.: 239,816

Related US. Application Data [62] Division of Ser. No. 804,470, March 5,I969.

[52] US. Cl 317/2 R, 250/49.5 TC, 317/262 A [5 1] Int. Cl. H05f 3/04[58] Field of Search 317/2 R, 2 A, 2 H,

317/3, 4, 262 A; 250/495 TC [56] References Cited UNITED STATES PATENTS3,474,292 10/1969 Carter 3l7/2 R Primary Examiner-James D. TrammellAssistant ExaminerHarry E. Moose, Jr. AttorneyRobert F. Cody [5 7]ABSTRACT A web with a plastic surface is dislcosed as being exposed toan a.c.-produced corona thereby to increase its printability andcoatability. As a result of such treatment the web surface undesirablyacquires patterns of charge. A circuit technique is disclosed for soerasing such charge patterns that the web surface is assuredly leftsubstantially uncharged.

6 Claims, 5 Drawing Figures BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates in general to web handling apparatusand methods; and in particular the invention provides new and improvedways to assure proper adhesion of materials such as coatings to plasticsurfaces; e. g. the surface of a plastic web; or the surface of aplastic coated web; etc.

2. Description Relative to the Prior Art It is well known that thesurfaces of synthetic plastics such as polyolefins (e.g. polyethylene,polypropylene and the like) and polyesters (e.g. polyethyleneterephthalate) are naturally resistant to printing and other coatings;and that the corona-treatment of such surfaces will minimize suchresistance. The amount of corona-current which flows through a surfacehas a direct bearing on the adhesion quality of such surface; and forsubstantial current to flow through a surface, such current should bealternating rather than direct because direct currents areself-extinguishing, i.e. they merely charge up a surface.

As used herein alternating currents and voltages are any such currentsand voltages which vary, such for example as sinusoidal and pulsatingcurrents and voltages.

In corona-treating a web by means of alternating voltages, it is usualthough not necessary to employ an array of treating electrodes, therebyto assure that the whole face of the web will be corona-treated.

FIG. 1 indicates this expedient; and is presented at this point in thespecification to facilitate an appreciation of the problem( s) which hasbeen solved by means of the invention.

FIG. I shows, in a partial side elevational view, a web 10 (e.g. paper)which is coated with a plastic 12 (e.g. polyethylene). The web 10 passesover a grounded roller 14, and moves in the direction indicated. Anarray of electrodes l6, 18, 20 corona-treat the plastic coating 12 withan alternating voltage (e.g. about 16 kv, at a distance d of 0.02inches); and such electrodes l6, 18, 20 cooperate toassure that thewhole face of the web is corona-treated. v

To dissipate any corona-produced voltage which may be stored on thecoating 12 of the web 10, it had been considered to roll up the web, orto ground both sides of same. It was found however that rolling the webleft it charged to about 250 volts; and grounding the web left itcharged to about 700 volts. While stored voltages of such magnitudescould be handled in many instances, it was found that in coating such acoronatreated web, the coating took on a striped pattern, rather than beevenly distributed across the whole face of the web. A study was made tofind out why the coating followed a particular pattern; and it wasdetermined that the last of the electrodes 20 to corona-treat the web Inot only charged the web in accordance with the aforementioned pattern,but often augmented such pattern by redistributing the charges placed onthe web by the other electrodes 16, 18. FIG. 2 is presented at thispoint in the specification to illustrate the effect of a charge patternon a web which is to be coated. The corona-treated web moves in theindicated direction past the mouth 21 of a coating hopper 22. Ideally,the

coating solution 26 should flow onto the web in the manner indicated bythe dotted line 24. However, because of the charge-pattern 23 producedby the electrode 20, ions within the coating solution 26 realign inaccordance with such charge-pattem, causing surface modulation of thecoating solution 26 as it leaves the mouth 21 of the hopper 22. Theamplitude of such modulation increases as the coating solution 26 nearsthe web 10 and, attendantly, the coating solution 26 gets deposited onthe web in accordance with the indicated charge pattern 23.

SUMMARY OF THE INVENTION It is the intent of the invention to apply astrong d.c. field to a web which has been corona-treated with analternating voltage. Such a dc. field does not coronatreat the webfurther; but serves merely to apply charges to those web areas whichhave little or no charge. In other words, any charge pattern which mighthave appeared on the web prior to its being d.c. flooded gets lostwithin such flood".

In many instances where the coating solution 26 is not a light-sensitiveone, stored web voltages such as the above-mentioned 250 and 700 voltsare not especially troublesome. When the coating solution 26 islight-sensitive, however, it is desirable to discharge the web prior toits reaching the coating hopper 22, whereby arcing will not occurbetween the web and the coating solution 26. One way to assure suchdischarge is in accordance with the invention to treat the websuccessively with equal d.c. fields, the first of one polarity and thesecond of the opposite polarity. While such a technique is useful, ithas been found further that the best way to assure equal intensity d.c.-treatment of a web is to have such fields formed as part of the sameseries-arranged circuit: A strong direct voltage is applied from asource, e.g. by means of a roller, to one face of the web; a firstelectrode disposed at the opposite face of the web sees such voltage,and such first electrode is electrically shorted to a second electrode.The second electrode is disposed closer to the hopper, but is on thesame side of the web as the first electrode. Opposite the secondelectrode, and in contact with the web, e.g. by means of a roller, isthe electrical return for the direct voltage source. With such anarrangement, whatever charges are applied to the web atone point thereoffor charge-pattern canceling purposes are-intrinsically, removed fromthe web prior to coating thereof; and so, the web has virtually nocharge at its coating point.

The above-described technique presupposes that the resistivity of theweb and its plastic coat is'greater than about 10 logohms per squarecentineter, ie.. that virtually no direct current flows within and alongthe web, and that all current entering at the first d.c.-treatment pointis returned to the source via the second d.c.- treatment point. Forapplication of the invention with respect to webs with resistivitiesless than about 10 logohms per square centimeters, a variation(s) of theabove-described series-disposed circuit is indicated.

An object of the invention is to provide improved apparatus and methodsfor treating plastic surfaces.

Another object of the invention is to provide an improved techniqueforremoving charge patterns from a plastic surface.

Another object of the invention is to provide apparatus and methods foruse in applying light-sensitive coatings to the plastic surfaces ofwebs.

The invention will be described with reference to FIGS. 3 through ofwhich FIG. 3 is a diagram illustrating how corona-producedcharge-patterns can be eliminated from a web, such web being so handledthat it can fairly easily be coated with a light-sensitive solution,

FIG. 4 is a diagram similar to the diagram of FIG. 3 but indicating howa procedure thereof may be obviated if the web-coating concerned is not(especially) light-sensitive, and

FIG. 5 illustrates a presently preferred technique(s) for so removingcorona-produced charge-patterns from a web that the web is leftsubstantially uncharged. whereby the web may be directly coated with alight-sensitive solution.

The corresponding parts of FIGS. 3 through 5 are all similarly numbered,the part numbers of FIGS. 4 and 5 being, however, primed anddouble-primed respectively.

Reference should be had to FIG. 3: A stock roll 28 ofpolyethylene-coated paper web 30 is fed over an electrically groundedroller 32. An alternating voltage is applied from a source 34 thereof toa plurality of electrodes 36, 38, 40. Such electrodes are so disposedopposite the grounded roller 32 that substantial aternating currents mayflow through, and thereby coronatreat, the web 30. As indicated above,the electrode 40, being the last to see any given point on the web,causes patterns of charges to appear and reside on the surface of theweb 30.

The web is passed over an electrically conductive roller 42, to which asource 44 of direct voltage (approximately 9,000 15,000 volts) isconnected. Such an expedient so deposits charges on the web 30 that thepatterns thereof disappear (although by this expedient the web is lefthighly charged). To reduce appreciably such charge (say to about 250volts, as indicated above), whereby the web 30 could fairly safely becoated with a light-sensitive coating, the web 30 is rerolled 48; afterwhich it is unrolled for coating by means of a hopper 50; and againre-rolled 52 after coat- To avoid the re-roll and unroll procedure ofFIG. 3, the charged web 30', after it is exposed to the dc. field at theroller 42, may be passed between electrically grounded rollers 54',whereby a fair amount of charge is bled off the web 30'. Such apractice, while solving one significant cost-consuming web handlingproblem, leaves the web fairly highly charged (about 700 volts asindicated above); and obviously such a web could not be coated with alight-sensitive solution very easily.

The method(s)/apparatus of FIG. 5 removes chargepatterns from acorona-treated web, without necessitating the re-roll and unroIlprocedure, and which method(s)/apparatus leaves the web substantiallyuncharged: FIG. 5 indicates a switch 60" which, in its position 1,provides the above features for webs of resistivities greater than aboutlogohms per square centimeter; for web resistivities less than about 10logohms per square centimeter, the switch 60" is placed in its position2.

In essence, the presently preferred application of the inventionindicates the disposition of electrodes 62" and 64" (which maythemselves be web rollers) opposite the charge and discharge rollers 42"and 54" respectivelyg and the electrical shorting of such electrodes bya lead 66". Thus, the charge-discharge circuit is turned into a seriescircuit, with the electrode 42" being electrically as much of onepolarity as the electrode 54" is of an opposite polarity. Since therollers 42" and 54" are connected electrically to the opposite terminalsof the source 46", the charge which is added to the web 30 by the roller42" is necessarily the same as the charge which is removed from the web30" by the roller 54".

For instances where the web 30" is a resistivity less than approximatelyl0 logohms per square centimeter, e.g. when such web 30 is provided orformed with an antistatic agent or coating, some leakage current mayflow from the source 46" through the web 30", and back to the source 46"without flowing via the electrodes 62" and 64". If such were to occur,the charge applied to the web via the roller 42" would probably begreater than that removed via the roller 54". To compensate for thischarge differential, the invention suggests that the switch 60 be closed(position 2), whereby a compensation current equal to the leakagecurrent may be applied through a currentcontrolling rheostat 70" to theelectrode 64".

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:

1. In a system for improving the adhesion characteristics of anelectrically non-conductive surface, said system including analternating voltage corona-producing power source, and means for use inexposing said surface to said corona, the improvement comprising a.first means for applying a direct voltage of one polarity to saidsurface after said surface has been exposed to said corona, and

second means for applying to said surface, subsequent to the applicationof voltage thereto by said first means, a direct voltage of a polarityopposite to that applied to said surface by said first means, thevoltages applied bysaid first and second means being approximatelythesame, wherebythe charge-patterns produced on said surface by saidalternating voltage power source are so erased that the said surface isleft substantially uncharged.

2. The system of claim I wherein a. said surface is the surface of aweb, and

b. said first and second means are so disposed that the charge appliedto said web by said first means is necessarily substantially the same asthat removed from the web by said second means.

3. The system of claim 2 wherein a. said web has a resistivity greaterthan approximately 10 logohms per square centimeter.

b. said first means comprises a first pair of electrodes disposed onopposite sides of said web, and

c. said second means comprises a second pair of electrodes disposed onopposite sides of said web,

and wherein said system includes d. means for electrically connectingtogether one electrode from each pair thereof, said connected electrodesbeing disposed on the same side of said web, and

6 e. a source of direct voltage, said source having first 6. A systemfor removing patterns of charge from a and second terminals, the firstterminal being elecweb comprising .trically connected to the freeelectrodes of one pair a. a first pair of electrodes disposed onopposite sides thereof, and the second terminal being connected of saidweb, to the free electrode of the other pair thereof. 5 b. a second pairof electrodes disposed on opposite 4. The system of claim 3 wherein atleast one of said sides of said web, first pair of electrodes, and atleast one of said second c. means for electrically connecting togetherone pair of electrodes is in direct contact with said web. electrodefrom each pair thereof, said connected 5. The system of claim 2 whereinsaid web has a resiselectrodes being disposed on the same side of saidtivity less than approximately 10 logohms per square 10 web, andcentimeter, and wherein said system includes means for d. a source ofdirect voltage having first and second compensating for charge currentswhich may flow terminals, the first terminal being electricallyconwithin said web, thereby to assure that the charges apnected to onefree electrode and the second termiplied to and removed from the web aresubstantially nal being connected to the other free electrode.identical.

1. In a system for improving the adhesion characteristics of anelectrically non-conductive surface, said system including analternating voltage corona-producing power source, and means for use inexposing said surface to said corona, the improvement comprising a.first means for applying a direct voltage of one polarity to saidsurface after said surface has been exposed to said corona, and b.second means for applying to said surface, subsequent to the applicationof voltage thereto by said first means, a direct voltage of a polarityopposite to that applied to said surface by said first means, thevoltages applied by said first and second means being approximately thesame, whereby the charge-patterns produced on said surface by saidalternating voltage power source are so erased that the said surface isleft substantially uncharged.
 2. The system of claim 1 wherein a. saidsurface is the surface of a web, and b. said first and second means areso disposed that the charge applied to said web by said first means isnecessarily substantially the same as that removed from the web by saidsecond means.
 3. The system of claim 2 wherein a. said web has aresistivity greater than approximately 10 logohms per square centimeter.b. said first means comprises a first pair of electrodes disposed onopposite sides of said web, and c. said second means comprises a secondpair of electrodes disposed on opposite sides of said web, and whereinsaid system includes d. means for electrically connecting together oneelectrode from each pair thereof, said connected electrodes beingdisposed on the same side of said web, and e. a source of directvoltage, said source having first and second terminals, the firstterminal being electrically connected to the free electrodes of one pairthereof, and the second terminal being connected to the free electrodeof the other pair thereof.
 4. The system of claim 3 wherein at least oneof said first pair of electrodes, and at least one of said second pairof electrodes is in direct contact with said web.
 5. The system of claim2 wherein said web has a resistivity less than approximately 10 logohmsper square centimeter, and wherein said system includes means forcompensating for charge currents which may flow within said web, therebyto assure that the charges applied to and removed from the web aresubstantially identical.
 6. A system for removing patterns of chargefrom a web comprising a. a first pair of electrodes disposed on oppositesides of said web, b. a second pair of electrodes disposed on oppositesides of said web, c. means for electrically connecting together oneelectrode from each pair thereof, said connected electrodes beingdisposed on the same side of said web, and d. a source of direct voltagehaving first and second terminals, the first terminal being electricallyconnected to one free electrode and the second terminal being connectedto the other free electrode.